There is proposed an image display apparatus in which a light emitting diode (LED) is used as the light emitting element. Such a type of image display apparatus is configured to include: a plurality of LEDs that emit each color of red (R), green (G), and blue (B); an illumination optical system into which lights from the plurality of LEDs are made to enter; a light valve having a liquid crystal display board into which the light from the illumination optical system enters; a color synthesis prism that synthesizes the light from the light valve; and a projection optical system for projecting the light from the color synthesis prism to a projection plane.
In an image display apparatus having the above-described configuration, it is desired that optical loss in the optical path from the light emitting element to the light valve is reduced as much as possible to increase the luminance of a projected picture.
Among the above-described components, the liquid crystal display board and the color synthesis prism have a polarization dependence, and to increase the efficiency of the optical system, it is desired that the light emitting element emit light in a uniform polarization state.
Moreover, as described in Non Patent Literature 1, there is restriction by etendue which is determined by the product of the area and the radiation angle of the light emitting element. That is, unless the value of the product of the light emission area and radiation angle of the light emitting element is made equal to or less than the value of the product of the area of incident surface of the light valve and an acceptance angle (solid angle) determined by the F number of the illumination optical system, the light from the light emitting element will not be utilized as the projection light.
For that reason, in an image display apparatus using LEDs, the issue is to reduce the optical loss described above without increasing the emission surface of the light emitting element in order to reduce the etendue of the emitted light of the light emitting element.
Patent Literature 1 (JP2009-111012A) discloses a semiconductor light emitting apparatus in which the surface orientation of the growth principal surface is prescribed for the purpose of achieving light emission having a large polarization ratio.
Patent Literature 2 (JP2007-109689A) discloses a light emitting element, which has as its object providing a light emitting element or the like capable of reducing etendue and supplying light with a high polarization conversion efficiency, and includes a light emitting section that is provided on a reference surface and supplies light, and a structure provided at the emission side of the light emitting section, wherein the structure includes a reflective polarizing plate that transmits polarized light in a first vibration direction, and reflects polarized light in a second vibration direction nearly orthogonal to the first vibration direction, and an optical section that transmits light from the reflective polarizing plate and is formed such that a refractive index changes periodically with respect to a two-dimensional direction substantially parallel with the reference surface.